130 Annals of the South African Museum. 



and until recently nearly all conchologists placed the species of both 

 Trigonephrus and Dorcasia in the genus Helix. Not until 1905 was 

 it discovered that these South African snails belonged to the Aca- 

 vidae, and this is one of the many important discoveries which 

 we owe to the wide researches of Dr. Pilsbry .* The resemblance 

 between the South African and European forms is purely super- 

 ficial : in their internal anatomy both Trigonephrus and Dorcasia 

 are far more primitive than any of the dart-bearing Helices. 

 This South African subfamily of the Acavidae may be named 

 the Dorcasiinae. 



But why are these snails only found in the extreme south-west 

 corner of Africa ? Since they occur in Madagascar and the Sey- 

 chelles on the one hand, and in Brazil on the other, we might have 

 expected that the Acavidae would have a much wider distribution 

 in Africa, and there can be little doubt that they once did inhabit 

 nearly half the continent. It is therefore necessary to explain why 

 their African distribution is now so limited. The reason is probably 

 this. 



Long after the advent of the Acaoidae, perhaps not until the 

 Cretaceous period, there arose in the tropics of Africa a new group 

 of snails, the Achatininae. The earliest members of this group were 

 small and slender, being scarcely distinguishable from the living 

 Stenogyrinae , but soon they grew amazingly, and adorned their 

 shells with flaming streaks of colour. Larger and larger and very 

 much broader became these snails until they resembled the Aca- 

 vidae, especially the Bulimiform members of the family. The 

 Acavidae were probably the only other large herbivorous snails 

 in Africa, and with this family the Achatininae would inevitably 

 come into competition. Now the Achatininae were a newer and 

 more highly organized group than the Acavidae, and some of them 

 became larger than any other snails on the face of the earth ; it is 

 therefore not surprising that the Acavidae gave way before them. 

 The Achatininae multiplied and spread in all directions, and 

 wherever they became abundant the Acavidae disappeared. They 

 were too late to get all the way across to South America before the 

 land-connection was broken by the sea ; but they spread as far as 

 St. Helena, and no members of the Acavidae are known to occur in 

 that island. In the east the Mozambique Channel prevented them 

 from invading Madagascar and the Seychelles, and the very few 

 species of Achatina which are now found in those islands have 

 probably made their way there comparatively recently. But there 



* Proc. Mai. Soc., 1905, vi. p. 287. 



